1. Field of the Invention
The present invention relates to a transconductance amplifier (hereinafter referred to as “Gm amplifier”) including a common-mode feedback circuit configured to determine a DC operating point of an output.
2. Description of the Related Art
A Gm amplifier utilizing differential amplification generally having a high differential mode gain has often a high common mode gain as well. In order to keep the Gm amplifier in a desired operation range, a common-mode feedback circuit configured to determine a DC operating point of an output is used. The DC operating point of the output is sometimes expressed as an output common voltage (for example, Japanese Patent Application Laid-open H05-226950), a DC bias potential (for example, Japanese Patent Application Laid-open 2005-286822), or a common potential (for example, Japanese Patent Application Laid-open 2010-273009).
FIG. 4 is an illustration of an example of a related-art Gm amplifier 200. The Gm amplifier 200 includes, in addition to a main-body Gm amplifier 202, a common-mode feedback circuit 201 configured to determine a DC operating point of the output of the Gm amplifier 202. The common-mode feedback circuit 201 has resistors 211 and 212 and an amplifier 221. The Gm amplifier 202 has transistors 241, 242, 251, and 252, a current source 231, input terminals 281 and 282, and output terminals 291 and 292. The input terminal 281 corresponds to a positive input INP, and the input terminal 282 corresponds to a negative input INN. Similarly, the output terminal 291 corresponds to a positive output OUTP, and the output terminal 292 corresponds to a negative output OUTN.
The input terminals 281 and 282 of the Gm amplifier 202 are connected to gate terminals of the transistors 241 and 242, respectively. Source terminals of the transistors 241 and 242 are connected together to one terminal of the current source 231. Another terminal of the current source 231 is connected to a power supply terminal VDD. A drain terminal of the transistor 241 is connected to the output terminal 292, a drain terminal of the transistor 251, and one terminal of the resistor 211. A drain terminal of the transistor 242 is connected to the output terminal 291, a drain terminal of the transistor 252, and one terminal of the resistor 212. Gate terminals of the transistors 251 and 252 are connected together to an output terminal of the amplifier 221 of the common-mode feedback circuit 201. Source terminals of the transistors 251 and 252 are connected together to VSS. A positive input terminal of the amplifier 221 of the common-mode feedback circuit 201 is connected to a node between the resistor 211 and the resistor 212. A negative input terminal of the amplifier 221 is connected to a reference voltage Vref1.
The operation of the Gm amplifier 202 is described. A voltage of the input terminal 281 is denoted by V(281), and a voltage of the input terminal 282 is denoted by V(282). When an input voltage difference V(281)-V(282) is positive, the bigger the magnitude of the input voltage difference is, the more the transistor 241 is controlled to be off and the transistor 242 is controlled to be on. A larger portion of the current of the current source 231 is distributed to the transistor 242 than to the transistor 241. When the input voltage difference V(281)-V(282) is negative, the bigger the magnitude of the absolute value of the input voltage difference is, the more the transistor 241 is controlled to be on and the transistor 242 is controlled to be off. A large portion of the current of the current source 231 is distributed to the transistor 241 than to the transistor 242.
In this case, when common-mode voltages of the output terminals 291 and 292 of the Gm amplifier 202 increases, a middle-point voltage of the resistors 211 and 212, which are connected between both the terminals, of the common-mode feedback circuit 201 also increases. When the middle-point voltage of the resistors 211 and 212 is larger than Vref1, the amplifier 221 performs control so that the transistors 251 and 252 turn on and the common-mode voltages of the output terminals 291 and 292 decrease. When the middle-point voltage of the resistors 211 and 212 is smaller than Vref1, the amplifier 221 performs control so that the transistors 251 and 252 turn off and the common-mode voltages of the output terminals 291 and 292 increase. In this manner, the common-mode feedback circuit 201 determines the DC operating point of the output of the Gm amplifier 200.
A current source connected to a differential amplifier circuit in the Gm amplifier generally has a high impedance, and hence the gain of the Gm amplifier is reduced when a resistor is connected to the output of the differential amplifier circuit. The reduction of the gain which is supposed to be obtained in design causes increase in input offset voltage of the Gm amplifier or other influences.